1. Field of the Invention
The present invention relates to a method of manufacturing an optical element having a function of suppressing the interface-reflected light amount, and to the optical element, and is suitable for optical elements of various types of optical devices including image pick-up devices such as cameras and video cameras, observation apparatuses such as telescopes and monocles, liquid crystal projectors, and optical scanning apparatuses of electrophotographic devices.
The present invention can easily provide a fine irregularity structure having a reflection preventing function on the whole surface of an optical substrate, and can provide an optical element having a favorable reflection preventing characteristic at a low temperature without thermally damaging the optical element, and hence, can be utilized for optical substrates having low heat resistance.
2. Description of the Related Art
A reflection preventing film composed of a thin film has conventionally widely been formed on an optical surface of optical elements such as lenses to reduce the optical reflection, utilizing light interference. In this case, the reflectance is a function of wavelength, and when the wavelength deviates from the designed central wavelength, the reflection reduction cannot be anticipated. Further, a film structure composed of two or more layers is adopted to enable reflection preventing films having broader bands and lower reflectance to be formed. However, in the case of multilayer films, when the wavelength deviates greatly from the designed central wavelength, the films exhibit in some cases higher reflectance than only a substrate. This is the case where incident light is perpendicular to a substrate, and in the case of inclined incident light, the thickness d is corrected by d/cos θ for the angle θ of the inclined rays to the substrate, thus exhibiting a different value from the value in the case where incident light is perpendicular to a substrate, and causing disadvantages in some cases.
In contrast with optical thin films utilizing light interference as in the above, many reflection preventing structures have recently been proposed in which a fine periodic structure of not more than the wavelength is formed on an optical surface without using an optical thin film.
Japanese Patent Application Laid-Open No. 2003-004916 discloses a window material of a display whose reflectance is reduced by providing a specific fine periodic structure on the surface of a transparent base material. The fine periodic structure has a shape in which the period PMAX of the most raised portion is set to be not more than the minimum wavelength λMIN of the visible light wavelength band in vacuum. The rate of the sectional area of the transparent base material in a horizontal sectional plane continuously gradually increases toward the most depressed portion from the most raised portion, and is set to be 1 at the most depressed portion.
The fine irregularity shape is fabricated by forming a resist layer on a glass substrate, and thereafter exposing and etching a pattern by electron-beam lithography or laser interferometry to corrode the glass substrate.
Japanese Patent Application Laid-Open No. H09-202649 proposes a petaloid transparent alumina film which is a transparent alumina film petaloidly randomly aggregated with crevices, obtained by applying a coating liquid including at least an aluminum alkoxide and a stabilizer on a base, baking the coated base at not less than 400° C. to form an amorphous alumina film, thereafter subjecting the film to hot water treatment, and only drying or drying and baking the treated film. It is described that when forming the petaloid random structure, a film can be provided having many crevices and a high specific surface area, which is useful for support materials, underlying films for water-repellent films and low reflection films.
Japanese Patent Application Laid-Open No. 2001-017907 discloses a low-temperature forming method in which a fine irregularity texture is formed on the surface of a base by applying a solution containing an aluminum compound on the base to form a film, and immersing the film in warm water without special baking. It also achieves an excellent water ultra-repellency state of a contact angle of not less than 150°, and a reflection preventing film exhibiting high transparency with a transmittance of not less than 90% in the visible region.
However, in the case of using a wet process such as a sol-gel method as in Japanese Patent Application Laid-Open No. H09-202649, since baking temperature at the time of forming a film is commonly as high as not less than 400° C., the process poses problems such that the process adversely affects the surface precision of optical substrates, and a film cannot be formed on a base material such as a resin base material unendurable to a high temperature.
On the other hand, there is a case where after a film is formed by applying a coating liquid containing a metal alkoxide containing at least aluminum and a stabilizer on an optical substrate as in Japanese Patent Application Laid-Open No. 2001-017907, if the film is immersed in warm water without specially being baked, it is dissolved out and the surface irregularity texture cannot sufficiently be formed, thus excellent reflection preventing performance is not obtained in same cases.
The present invention has been made in consideration of such related arts, and the object of the present invention is to provide an optical element in which a fine irregularity structure having a sufficient reflection preventing function can be created even by forming a film on the optical substrate surface at a low temperature from a coating liquid containing a metal alkoxide containing at least aluminum and a stabilizer and which has a good reflection preventing characteristic.